The present invention is directed to signal amplifier devices, and especially to differential signal amplifier devices. Supply voltages in amplifier devices are decreasing as efforts to create products having lower power requirements continue. Lower power requirements contribute to longer battery life, less heat generation and other advantages that are particularly useful in small, compact products. Differential signal amplifiers are experiencing a trend toward lower supply voltages, especially as differential signal amplifiers are increasingly manufactured to meet the Low Voltage Differential Signaling (LVDS) and Low Voltage Positive Emitter Coupled Logic (LVPECL) standards. Performance of amplifier devices, including LVDS and LVPECL amplifiers exhibits significant variation in various operational parameters as supply voltage varies.
If a particular amplifier is employed using a variety of supply voltages, there may be unacceptable variance in such operational parameters as gain, output voltage (e.g., output differential voltage, Vod, in LVDS and LVPECL amplifiers), and edge rate (a measure of definition of a signal edge). By way of example and not by way of limitation, as supply voltage drops, bias current drops. As a consequence of lower bias current, gain drops, Vod is reduced and speed is reduced. As gain is reduced, edge rate deteriorates.
There is a need for an apparatus and method to operate an amplifier device at substantially constant operational parameters over a range of supply voltages. In particular, there is a need for an apparatus and method for compensating operating current in an amplifier when supply voltage varies.